Elsevier

Electrochimica Acta

Volume 283, 1 September 2018, Pages 1261-1268
Electrochimica Acta

Excellent room-temperature performance of lithium metal polymer battery with enhanced interfacial compatibility

https://doi.org/10.1016/j.electacta.2018.07.040Get rights and content

Abstract

Recently, rechargeable lithium metal polymer battery (LMPB) attracts increasing attention because of its high safety and energy density properties. However, the high contact interfacial resistance of solid-state electrode and electrolyte is still an obstacle to satisfy the demand of high current density and long cycle stability, especially at room and lower temperature. Here, we develop a simple and efficient heat treatment method, to improve the interfacial compatibility between electrode and electrolyte in all-solid-state LMPB, successfully realizing workable LMPB at a reduced temperature (30 °C). The LMPB shows excellent cycling performance and rate capability in the voltage range of 2.5–4.2 V. The behavior of Li+ plating/stripping on the surface of lithium metal anode is greatly enhanced. Meanwhile, the interfacial resistance of the LMPB decrease significantly after the heat treatment process.

Introduction

Lithium metal polymer battery (LMPB), using lithium metal as anode and dry polymer electrolyte film instead of inflammable organic liquid electrolyte, shows broad prospect in portable electronic devices, flexible electronic devices, electric vehicles, and energy storage of power grid. Due to the high energy density and safety, LMPB has attracted much concerns and been extensively investigated [[1], [2], [3], [4]]. Generally, poly (ethylene) oxide (PEO)-based solid-state polymer electrolytes exhibits satisfactory ions conductivity and good interfacial wetting effect at a middle-high temperature range beyond the melting temperature (Tm). Therefore, PEO and its derivatives were regarded as one of the most desirable matrix material candidates for polymer electrolytes [5,6].

It has been demonstrated that the LMPBs with PEO-based solid-state polymer electrolytes cycling at high temperature range of 50–120 °C could ensure the high ions conductivity and good interfacial compatibility [[7], [8], [9], [10], [11]]. However, high temperature will narrow the electrochemical stability window (<4 V vs. Li/Li+), limiting the practical applications of high-voltage positive electrodes in LMPB [12,13]. Meanwhile, elevated temperature will decrease the mechanical strength of the polymer electrolytes and encourage the growth of metal lithium dendrites [14]. Therefore, operation in the room or near room temperature range is necessary for the large-scale applications of the LMPB in the future.

Generally, in the PEO polymer electrolytes, the amorphous phase polymer chain, displaying good flexibility above the glass transition temperature (Tg), is responsible for the Li+ transport in the electrolyte [5,15]. However, PEO polymer matrix will present high crystallization at room temperature, deeply limiting the migration rate of the Li+ [16]. In order to relief the contradiction, liquid plasticizers, such as carbonate solvents [17], room temperature ionic liquids [18], and plastic crystal [19], are usually used to effectively reduce the crystallinity of the electrolytes and increase the proportion of amorphous phase. Unfortunately, liquid or plastic crystal components will weaken the mechanical strength and thermal stability of the polymer electrolyte, leading to unpredictable security issues. Different from liquid plasticizer, dispersing the passive ceramic filler (SiO2 [20,21], ZnO [22,23], Al2O3 [24,25]) or active ceramic filler (Li0.35La0.55TiO3 (LLTO) [26], Li7La3Zr2O12 (LLZO) [27,28], Li1.5Al0.5Ge1.5(PO4)3 (LAGP) [29,30], and Li10GeP2S12 (LGPS) [10]) into polymer matrix can actually increase mechanical property. At the same time, ceramic fillers can play an important role in Li+ transport across the electrolyte, where the Lewis acid-base interactions among filler surface and lithium salt as well as polymer chain can effectively suppress the transfer of anion [27]. Therefore, the composite solid-state electrolytes usually exhibit high ionic conductivity and enhanced lithium ion transference numbers (TLi+). As a result, excellent electrochemical performance will be achieved related with the synergistic effect of organic and inorganic electrolyte [31].

In fact, although the composite solid-state electrolyte shows considerable mechanical strength, excellent ionic conductivity and TLi+, the electrochemical performance of the LMPB still could not cycle stably under high current density at room temperature due to the high interfacial resistance, whichresults from complex solid/solid interfaces including cathode/electrolyte, lithium anode/electrolyte, active materials/electrolyte/conductive agent in cathode. Especially under room temperature, PEO have lower fluidity and flexibility than high temperature (50–120 °C) [[32], [33], [34]]. All above factors cause high interface charge transfer resistance and large polarization potential, resulting in a poor electrochemical performance at room temperature. Literature about LMPB with excellent electrochemical properties (especially rate capability) at room temperature is seldom reported so far.

In this work, we develop a simple heat treatment approach to improve the interfacial compatibility, leading to a favorable interface kinetics and therefore satisfactory electrochemical performance at the reduced operating temperature (30 °C). The influence of heat treatment on the electrochemical performance of Li-Li symmetric batteries and Li-LFP full batteries were systematically investigated, where interfacial resistance and Li+ plating/stripping behavior of the Li/CSSE@20/Li symmetric batteries with or without heat treatment were systematically analyzed by Scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS). After heat treatment, Li-Li symmetric battery showed lower impedance value at the open circuit potential. Meanwhile, the surface of lithium metal anode exhibited high smoothness without obvious dendrites growth after 200 cycles. Heat treatment approach is also an effective way to reduce the cathode/electrolyte interfacial resistance, and provide a stable pathway for Li+ transport in the cathode bulk phase.

Section snippets

Preparation of composite solid-state electrolyte and LMPB

The organic-inorganic composite solid-state electrolyte film was prepared by solution casting method. PEO (5 × 106 average molecular weight, Aladdin) and battery-grade LiN(CF3SO2)2 (LiTFSI, ≥99.0%, Aladdin) were dried under vacuum for 24 h at 60 °C and 120 °C for subsequent experiments. PEO and LiTFSI (EO: Li = 18: 1, molar ratio) were mixed into anhydrous acetonitrile (ACN) solvent and stirred for 12 h, and the obtained homogeneous solution was marked as PLA. Then LAGP (particle

Composite solid-state electrolyte

Solid-state electrolytes are the crucial factor for the electrochemical performance of LMPB, especially the cycling performance and rate capability. For lithium ion transport pathway in composite solid-state electrolytes, it has been demonstrated that a high ion conductive interphase, reported as a continuous percolation pathway [35], will be formed between active filler and polymer electrolyte matrix, with an appropriate ceramic filler loading [35,36], which will accelerate the lithium

Conclusions

In summary, we proposed a facial and effective heat treatment method, to reduce the interfacial resistance of electrode/electrolyte. As a result, the Li/CSSE@20/Li symmetric battery shows homogeneous Li+ plating/stripping behavior on the surface of lithium metal anode. Moreover, LMPB also shows excellent electrochemical performance including long cycling stability and favorable rate capability operating at a reduced temperature (30 °C). This work has provided a novel approach for the future

Acknowledgments

The work was supported by National Natural Science Foundation of China (Grant No. 21373072 and No. 51202047). The authors also thank the Center of Analysis and Measurement of Harbin Institute of Technology.

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